1. Field of the Invention
The present invention relates to a liquid crystal display device in which a liquid crystal material having spontaneous polarization is provided, for displaying images by on/off driving of switching elements.
2. Description of the Related Art
Along with the recent development of a so-called information-oriented society, electronic apparatus represented by personal computers, PDA (Personal Digital Assistant), etc. have come to be popularly used. As such electronic apparatus are gaining popularity more demand for portable type apparatus that can be used in an office as well as outdoors is arising, which is further creating an increasing demand for more compact and less weighing apparatus. As one of the means for meeting such a demand liquid crystal display devices are being widely used. A liquid crystal display device represents an essential technique for not only making products smaller in dimensions and lighter in weight but also for lowering power consumption of those portable type electronic apparatus driven by batteries.
Liquid crystal display devices can be classified broadly into a reflection type and a transmission type. A reflection type liquid crystal display device is so constituted that rays incidenting through the front face of liquid crystal display panel reflect from the rear face thereof, and such reflecting light makes images visible, while in a transmission type liquid crystal display device, transmitted light from a light source (back-light) provided on the rear face of liquid crystal display panel makes images visible. Since the reflection type is inferior in visibility because reflecting light amount is inconstant as it depends on circumstantial conditions, transmission type liquid crystal display devices are generally used especially for display devices of personal computers etc. for displaying multicolored or full-colored images.
Now, color liquid crystal display devices currently in popular use are generally classified into an STN (Super Twisted Nematic) type and a TFT-TN (Thin Film Transistor-Twisted Nematic) type. The STN type can be manufactured at a relatively low cost, however the disadvantage is that it is prone to crosstalk and slow in response, due to which this type is not suitable for displaying moving images. On the other hand, the TFT-TN type has a higher display quality compared to the STN type, however a back-light of a high luminance is required because light transmittance of a liquid crystal display panel is only around 4% at the current stage. Accordingly, since the TFT-TN type consumes much power for the back-light, it is not appropriate for use in an apparatus with portable batteries. Also, since colors are displayed by color filters a pixel must be constituted by three sub-pixels for red/green/blue colors, therefore it is difficult to achieve a high resolution, and besides, the purity of displayed colors is not satisfactory.
With an object to solve the foregoing problems, the inventors of the present invention have developed a liquid crystal display device of field-sequential method. The liquid crystal display device of field-sequential method does not require sub-pixels, and therefore a display with a higher resolution can be easily achieved compared to a liquid crystal display device with color filters, and moreover since colors emitted from a light source are directly used for display without using color filters, the purity of displayed colors is superior. Further, the liquid crystal display device of field sequential method achieves a higher light utilization efficiency, and therefore has another advantage of low power consumption. However, a high-speed response of liquid crystal is an essential factor for accomplishing a liquid crystal display device of field-sequential method. From such a viewpoint, the inventors of the invention are developing a technique of driving, by a switching element such as a TFT etc., liquid crystal such as a ferroelectric liquid crystal etc. that has spontaneous polarization, from which 100 to 1,000 times quicker response can be expected than conventional devices, with an object to achieve a high-speed response in liquid crystal display device of field-sequential method that has the aforementioned advantages, also in liquid crystal display device of color filter method.
In a ferroelectric liquid crystal, a direction of major axis of a liquid crystal molecule is changed by 2 θ by an application of voltage, as shown in FIG. 1. A liquid crystal display panel provided with a ferroelectric liquid crystal enclosed between a pair of substrates is placed between two polarizers that have orthogonal polarizing axis, and intensity of transmitted light is varied utilizing birefringence caused by changes of major axis direction of liquid crystal molecules. When ferroelectric liquid crystal is driven by a switching element such as a TFT, switching of spontaneous polarization is caused in proportion with an amount of charge loaded on (stored in) a pixel through the switching element, and thus the intensity of transmitted light is changed.
In a conventional liquid crystal display device wherein a liquid crystal such as a ferroelectric liquid crystal that has spontaneous polarization is driven by a switching element such as a TFT, 2Ps·A (total charge amount of switching current resulting from a complete reversal of spontaneous polarization), where Ps is a size per unit area of spontaneous polarization and A is electrode area of each pixel, is not more than a charge amount Q loaded on each pixel through the switching element. In other words, liquid material, pixel electrodes, a TFT, etc. are designed so as to satisfy a condition of 2Ps·A≦Q.
In conventional devices, a maximum intensity of transmitted light is obtained by a complete reversal of spontaneous polarization under the condition of 2Ps·A≦Q, and therefore, when the driving voltage is as low as 7V or less, a size of spontaneous polarization Ps that satisfies the mentioned formula can only be as small as 8 nC/cm2 or less due to the limited capacity of liquid crystal, and consequently responses become slow since Ps cannot be sufficiently large. Therefore it is necessary to increase the size of spontaneous polarization from a viewpoint of responsiveness, especially responsiveness at a low temperature. However, when a liquid crystal material with a large spontaneous polarization is to be employed because of responsiveness and availability of applicable liquid crystal materials, another problem arises because Q must be a large value and a high driving voltage is therefore required.